1. Field of the Invention
The present invention relates to a position control device, a measuring device and a machining device. In particular, the present invention relates to a position control device that controls a position of a driven body driven by a driver according to a predetermined position command, as well as to a measuring device and a machining device each including the position control device.
2. Description of Related Art
Conventionally, there has been known a servomechanism that controls a position of a driven body driven by a motor etc. according to a predetermined position command (for instance, see Document 1: JP-A-2004-118635). The servomechanism is utilized for a NC (numerical control) coordinate measuring machine, a NC machine tool etc.
To drive and control a low rigid load (a driven body) causing vibrating response to a displacement, a speed and an acceleration thereof since a low rigid portion is provided at a connection with the motor for instance, the servomechanism has been developed, which includes a quadruple loop control system having a speed control loop that controls a speed of the load, in addition to a current control loop, a motor speed control loop and a position control loop that controls a position of the load (see aforementioned Document 1).
FIG. 25 shows an arrangement of the conventional servomechanism 10.
In FIG. 25, the current control loop and the motor speed control loop are grouped as a drive controller 50 having a transfer function Gm. Note that the drive controller 50 outputs a motor speed Vm as an output from the motor speed control loop. And, a transfer characteristic of the driven body 20 is defined as Gf.
Though detail arrangement is omitted, the current control loop has a motor, a motor drive power amplifier, a motor torque current detector and a current characteristic compensator, whereas the motor speed control loop has a motor rotation position detector that detects a rotation position of the motor, a differentiator that calculates a rotation speed of the motor by differentiating the rotation position of the motor, and a motor speed characteristic compensator.
The speed control loop 30 includes a differentiator 330 that differentiates the position Pd of the driven body 20 to calculate the speed Vd of the driven body 20, a speed comparator 310 that compares a speed command Vdr output from the position control loop 40 with the speed Vd of the driven body 20 calculated by the differentiator 330 to output a speed error Evd=Vdr−Vd, and a speed characteristic compensator 320 having a transfer function Glv. Note that an output from the speed characteristic compensator 320 is a motor speed command Vmr for the motor speed control loop.
The position control loop 40 includes a position comparator 410 that compares the current position Pd of the driven body 20 with a predetermined position command Pdr to output a position error Epd=Pdr−Pd, and a position characteristic compensator 420 having a proportional gain K. Note that reference numeral 430 in FIG. 25 represents an integrator element having a transfer function 1/s (“s” is Laplace operator), which integrates the speed Vd of the driven body 20 to calculate the position Pd of the driven body 20.
By having the speed control loop 30, vibration suppressiveness of a control system can be enhanced even when rigidity of the driven body 20 is low, thereby controlling the position or the speed of the driven body 20 stably and highly accurately.
According to the servomechanism 10, any transfer characteristic can be provided by setting the respective transfer functions and gains to appropriate values. The respective transfer functions and gains can be set for a variety of purposes. For example, the respective transfer functions and gains can be set to restrict a torque ripple of the motor included in the drive controller 50 and a vibrating behavior of the driven body 20 caused by a disturbance such as a frictional force acting on the driven body 20, or to avoid an overshoot of the position Pd and speed Vd of the driven body 20 relative to the position command Pdr and the speed command Vdr.
The transfer characteristic of the servomechanism 10 is discussed below.
To simplify the description, the discussion is held on the presumption that a response frequency of the drive controller 50 (the transfer function Gm) and a resonant frequency of the driven body 20 (a transfer function Gf) are sufficiently higher than a break frequency of the speed characteristic compensator 320 (the transfer function Glv) in the speed control loop 30. According to the presumption, Gm and Gf can be approximated as Gm≈1 and Gf≈1, thus the servomechanism 10 of FIG. 25 can be redrawn as the one shown in FIG. 26. Then, when equivalently transforming the speed control loop 30 of FIG. 26, the arrangement in FIG. 28 can be obtained through the arrangement in FIG. 27. If further equivalently transforming the arrangement in FIG. 28 (not shown), a transfer function Gc from the position command Pdr to the position output Pd can be expressed by an Equation (1) as follows.
                    Gc        =                  1                      1            +                                          (                                  1                  +                                      1                    Glv                                                  )                            ⁢                              s                K                                                                        Equation        ⁢                                  ⁢                  (          1          )                    
As expressed in the Equation (1), the transfer function Gc includes the transfer function Glv of the speed characteristic compensator 320 in the speed control loop 30.
Incidentally, when setting Glv for suppressing the vibrating behavior of the driven body 20 in the above-described servomechanism 10 having the transfer characteristic Gc including Glv, there may be occurred the overshoot. This is because not all the set Glv for suppressing the vibrating behavior are appropriate for preventing the overshoot of the position Pd and the speed Vd of the driven body 20 from occurring relative to the position command Pdr and the speed command Vdr. In addition, when the Glv is set for preventing the overshoot from occurring, the driven body 20 may vibrate since not all the set Glv are appropriate for suppressing the vibrating behavior of the driven body 20.